A first-of-its-kind study yields clues to how some people are able to survive the deadly Ebola virus and suggests possible avenues for treatments that could save more lives.

“These findings are encouraging and underscore how crucial it is to continue the fight against Ebola,” says Tom Frieden, director of the Centers for Disease Control. “We must come up with new ways to keep people safe and combat diseases that threaten our health.”

Ebola virus disease (EVD) causes a severe and often fatal infection that can include fever, diarrhea, and unexplained bleeding. Despite the fact that more than 30,000 people have acquired Ebola virus disease since it was discovered in 1976, the medical and scientific community still does not have a clear understanding of the mechanisms by which it causes such severe illness.

This is the first time researchers have been able to study the virus using samples taken from patients during both their illness and recovery. In the seven patients from the United States, researchers tracked 54 different markers of immune-system activity from hospital admission until the day of discharge. Among the seven patients, five had moderate EVD and two had severe EVD requiring mechanical ventilation and dialysis.

“We were able to identify the particular components of patients’ immune systems that successfully fought off the virus,” says lead author Anita McElroy, a guest researcher in CDC’s viral special pathogens branch and a physician and faculty member in the pediatrics department (infectious diseases) at Emory University School of Medicine. “These are the parts of the immune system that we need to tap into to develop new therapies.”

The results, published in the Clinical Infectious Disease Journal, show patients with severe EVD have high levels of virus in their blood and out-of-control immune responses leading to destruction of healthy tissues, multisystem organ failure, shock, and, in most cases, death. In contrast, patients with moderate EVD have strong, healthy immune responses that were able to control the virus. All of the patients with moderate illness and one patient with severe illness survived.

It is unclear why exactly some people’s immune systems respond more effectively to viruses. Possible contributing factors include genetics and whether or not a person has other illnesses or conditions. Identifying which parts of the immune system malfunction in severe EVD cases as well as the parts that function well in the moderate cases could help .design therapies that might theoretically inhibit progress of the disease.

Other researchers from Emory and from the University of Nebraska Medical Center are coauthors of the study.